Driving apparatus for a light emitting device and method for the same

ABSTRACT

A driving apparatus configured to drive a light emitting device includes a driving current source module operable to supply current to the light emitting device via a node during operation. A protection module coupled to the node and the driving current source module selectively injects current to the node during operation. The driving current source module is controlled based on a detection result of a voltage on the node.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application from U.S. patent Ser. No.15/486,652 filed Apr. 13, 2017, which is a divisional application fromU.S. patent Ser. No. 14/966,662 filed Dec. 11, 2015, now U.S. Pat. No.9,661,701, which is a divisional application from U.S. patent Ser. No.14/454,867 filed Aug. 8, 2014, now U.S. Pat. No. 9,370,059, which claimspriority from Chinese Application for Patent No. 201310361941.3 filedAug. 9, 2013, the disclosures of which are incorporated by reference.

TECHNICAL FIELD

Embodiments generally relate to the field of photoelectric technology,and more specifically to a driving apparatus for driving a lightemitting device and a method for the same.

BACKGROUND

Light emitting devices such as light emitting diodes (LED) have beenprevalently applied nowadays. Generally, a light emitting device isdriven by a corresponding driving apparatus, e.g., referring to FIG. 1,one end of the light emitting module 1 is provided a voltage V_(B) foroperation, while the other end is connected to a driving module 2 so asto be driven by the driving module 2.

FIG. 2 is another example of a light emitting device and a drivingapparatus in the prior art, wherein the light emitting device is a LEDarray such as active matrix organic light emitting diode (AMOLED), whilethe driving apparatus is a driving current source module. In FIG. 2, theLED array has a plurality of LED strings, e.g., LED strings L11, L12, .. . , L1 n-1 and L1 n, and LED strings Lm1, Lm2, Lmn-1 and Lmn. Similarto FIG. 1, a boost module 3 provides voltage VB to each LED string, andeach LED string is driven by driving current source modules 21, 22, . .. , 2 m.

On one hand, light emitting devices such as LED array often fail. FIGS.3A and 3B show two categories of faults, wherein for the convenience ofillustration, only one LED string is shown, while other LED strings areomitted. For example, with reference to FIG. 3A, one category of faultsis that one end of the LED string connected in series with the drivingcurrent source module is grounded, which causes an uncontrollabledriving current source module, and much worse, it will even damage theLED array and the driving current source module. The other category offaults is that an open circuit occurs in the LED string. For example,with reference to FIG. 3B, the open circuit (indicated with an “X”)occurs between the voltage V_(B) provided by the boost module and theLED L11. It may be understood that the open circuit may also occurbetween for example L11 and L12, or L1 n-1 and L1 n, or L1 n and thecurrent source. The prior art always employs a complex and independentcircuit module to detect the two categories of faults, which raises thecost and makes circuit design more complex. Therefore, there is a needfor a driving apparatus which has a simple design and can detect atleast a part of the above faults and a corresponding detecting method.

On the other hand, more and more LED applications require that the LEDlight be dimmable, which requires that the LED driver have differentdriving capabilities according to the needs so as to change theintensity of the light emitted by the LED. Besides, in a combination ofLEDs emitting different colors of light, wherein different colors aremixed to obtain light with a particular light temperature, it is neededthat drivers for LEDs of one or more colors can dim light. In alow-light state, a conventional LED driver generally uses a pulse widthmodulation (PWM) to dim light. At this point, delay always occursbetween the PWM signal and the dimming current in the driver, whichcauses dimming delay, such that the driving current of the LED tends tobe inaccurate, which causes the LED light to be overly dimmed and thebrightness of the LED panel to be inaccurate. Therefore, a drivingapparatus improving response and an improved method are desirable.

SUMMARY

Embodiments intend to at least partially solve or alleviate the aboveproblems.

According to one aspect, there is provided a driving apparatus fordriving a light emitting device, comprising: a driving current sourcemodule configured to supply current to the light emitting device via anode during operation; a protection module coupled to the node and thedriving current source module and configured to selectively injectcurrent to the node during operation and control the driving currentsource module based on a detection result of a voltage on the node.

According to another aspect, there is provided a display system,comprising a light emitting device and the above driving apparatus fordriving the light emitting device.

According to a further aspect, there is provided an electronic deviceincluding the above display system.

According to a still further aspect, there is provided a method forprotecting a display system, which display system comprises a lightemitting device and a driving apparatus for driving the light emittingdevice, which light emitting device is coupled to a current source ofthe driving apparatus via a node, which method comprises: disabling thedriving current source module before startup of the driving apparatus;injecting current into the node; detecting a voltage on the node; anddetermining whether to enable or disable the driving apparatus based onthe detected voltage.

According to a yet further aspect, there is provided a method forprotecting a display system, which display system comprises a lightemitting device and a driving apparatus for driving the light emittingdevice, which light emitting device is coupled to a current source ofthe driving apparatus via a node, which method comprises: detectingvoltage on the node when the driving apparatus is running; disabling thedriving current source module when it is detected that voltage of thenode is less than a threshold voltage; injecting current into the node;detecting a voltage on the node; and selecting only disabling thedriving current source module or disabling the entire driving apparatusbased on the detected voltage.

According to a still further aspect, there is provided a drivingapparatus for driving a light emitting device, comprising: a drivingcurrent source module configured to supply current to the light emittingdevice via a node during operation, wherein the driving current sourcemodule is configured to improve dimming accuracy by reducing delaycaused by an operational amplifier included in the driving currentsource module when dimming in a pulse width modulation (PWM) mode.

According to a still further aspect, there is provided a display system,comprising a light emitting device and the above driving apparatus fordriving the light emitting device.

According to a still further aspect, there is provided an electronicdevice comprising the display system.

According to a still further aspect, there is provided a method fordriving a light emitting device, comprising: improving dimming accuracyby reducing delay caused by an operational amplifier in the drivingapparatus when dimming in a pulse width modulation (PWM) mode.

In an embodiment, an apparatus comprises: a driving current sourcemodule including an operational amplifier coupled to a drive transistorthat is configured to supply current to a light emitting device via anode; and a circuit included in the driving current source module thatis configured to improve dimming accuracy by reducing delay caused bythe operational amplifier upon dimming in a pulse width modulation (PWM)mode.

In an embodiment, an apparatus comprises: a drive transistor having afirst conduction terminal, a second conduction terminal and a controlterminal; an operational amplifier having a first input, a second inputand an output; a first switch coupled between the second input and thesecond conduction terminal; a second switch coupled between the outputand a reference node; wherein the first and second switches are actuatedin response to a pulse width modulation (PWM) control signal; a firstresistive circuit coupled between the second conduction terminal and thereference node, wherein the first resistive circuit has a variableresistance value; a second resistive circuit coupled between the firstinput and the reference node; and a control circuit configured to changethe variable resistance value in response to a change in logic state ofthe PWM control signal.

In an embodiment, an apparatus comprises: a drive transistor having afirst conduction terminal, a second conduction terminal and a controlterminal; an operational amplifier having a first input, a second inputand an output; a first switch coupled between the second input and thesecond conduction terminal; a second switch coupled between the outputand a reference node; wherein the first and second switches are actuatedin response to a pulse width modulation (PWM) control signal; a firstresistive circuit coupled between the second conduction terminal and thereference node; a second resistive circuit coupled between the firstinput and the reference node, wherein the second resistive circuit has avariable resistance value; and a control circuit configured to changethe variable resistance value in response to a change in logic state ofthe PWM control signal.

In an embodiment, an apparatus comprises: a drive transistor having afirst conduction terminal, a second conduction terminal and a controlterminal; an operational amplifier having a first input, a second inputand an output; a first switch coupled between the second input and thesecond conduction terminal; a second switch coupled between the outputand a reference node; wherein the first and second switches are actuatedin response to a pulse width modulation (PWM) control signal; andwherein the operational amplifier includes a differentialtransconductance stage and a circuit configured to reduce delay byincreasing current flowing through the differential transconductancestage in response to a logic state change of the PWM control signal.

A corresponding advantageous effect may be achieved by using someembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Some exemplary embodiments are illustrated in the accompanying drawingsthat are not drawn in proportion. In the accompanying drawings, similarreference numerals represent similar components, wherein:

FIG. 1 is an exemplary example of a display system in the prior art;

FIG. 2 is an exemplary example of a LED panel in the prior art;

FIGS. 3A and 3B are examples of short-circuit fault and open-circuitfault in the LED panel, respectively;

FIG. 4 is an example of a driving apparatus that has a protection moduleand is configured to drive a light emitting device according to oneembodiment;

FIG. 5 is an example of a driving apparatus that has a protection moduleand is configured to drive a light emitting device according to oneembodiment;

FIG. 6A is a flow chart of an example of a method for detecting a faultand protecting a driving apparatus according to one embodiment;

FIG. 6B is a flow chart of an example of a method for detecting a faultand protecting a driving apparatus according to another embodiment;

FIG. 7 is an example of a driving apparatus that has a protection moduleand is configured to drive a light emitting device according to oneembodiment;

FIG. 8 is an example of a driving apparatus for driving a light emittingdevice according to one embodiment;

FIG. 9 is an example of a driving apparatus according to one embodiment;

FIG. 10 is an example of delay between a PWM signal and a drivingcurrent;

FIG. 11 is a circuit diagram of an operational amplifier of a drivingapparatus;

FIG. 12 is a circuit diagram of an operational amplifier of a drivingapparatus;

FIG. 13 is a diagram of a simulation result of the embodiment accordingto FIG. 12; and

FIGS. 14A and 14B are examples of a resistance unit according to oneembodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

Some specific details will be expounded below so as to provide thoroughunderstanding on various aspects of the subject matter as disclosed.However, the disclosed subject matter can also be implemented withoutthese specific details. In some embodiments, known structures andmethods for forming a structure associated with a semiconductor deviceare not described yet so as to avoid ambiguity of the description ofother aspects of the present disclosure.

Unless otherwise required in the context, the term “comprise” appearingin the description and the whole text of the appended claims will beinterpreted as open inclusion, i.e., interpreted as “including, but notlimited to.”

In the whole text of the present description, reference to “anembodiment” or “embodiments” means the specific features, structures orcharacteristics described in conjunction with the embodiment(s) areincluded in at least one embodiment. Therefore, the phrases “in anembodiment” or “in the embodiment” appearing in various parts throughoutthe whole text of the present description do not necessary refer to thesame aspect. Besides, particular features, structures or characteristicsmay be combined in any appropriate manner in one or more aspects of thepresent disclosure.

Now, refer to FIG. 4, in which an arrangement according to oneembodiment is presented. In the arrangement, for the sake ofillustration, only one LED string is illustrated. Those skilled in theart may understand that more LED strings may be included therein, andthe LED string may also be other light emitting device suitable for thisoccasion.

In FIG. 4, the LED string is powered by a boost module 3 and is coupledin series to a current source via a node N_(cs). Those skilled in theart may understand that the node N_(cs) may be any point between the LEDconnected to the current source 21 in the LED string and the currentsource, for example, when the current source is integrated in thedriving apparatus while the LED string is integrated into the lightemitting device, the node N_(cs) may be a pin of the driving apparatusconnected to the light emitting device.

Those skilled in the art may understand that the boost module 3, theprotection module 4, and the driving current source module 21 in FIG. 4may be integrated into an integrated circuit chip or located in adifferent integrated circuit chips.

As shown in FIG. 4, in one embodiment, the protection module 4 isintegrated into the driving apparatus, so as to, for example, be usedfor detecting two categories of faults as stated above and perform acorresponding protection action. The protection module 4 determines aresult of detecting based on a signal from the node N_(cs), andselectively disables the current source 21 or the driving apparatusboost module 3. For example, when it is determined that short circuitexists in the LED string, in order to avoid the current source 21 frombeing uncontrollable and potential damage of the driving apparatus andlight emitting device, the whole driving circuit may be disabled. Forexample, when it is determined that open circuit occurs in the LEDstring, it is allowed to only disable the driving current source module21 corresponding to the LED string.

Those skilled in the art may understand, disable means stopping use of acorresponding device or apparatus. For example, in the case of disablingthe current source 21, the current source may be made not to operate bya control signal, or no power is supplied to the current source, or thecurrent source is disconnected from other device, apparatus or circuitand the like so as to be electrically isolated from other device,apparatus or circuit and the like.

Hereinafter, operation of a protection module according to oneembodiment will be illustrated specifically with reference to FIG. 5,FIG. 6A and FIG. 6B, wherein FIG. 5 is a specific example of aprotection circuit 4 coupled to the current source 21 and the boostmodule 3, FIG. 6A is an example of a method of detecting a fault beforestartup of the light emitting device such as LED panel, while FIG. 6B isan example of a method of detecting a fault when a light emitting devicesuch as LED panel is operating.

First, with reference to FIG. 5 and FIG. 6A, before startup of a lightemitting device such as LED panel, it is desirable that a fault in thelight emitting device may be detected so as to avoid potential damage ofthe device upon startup. For the light emitting device, it is paramountto avoid a situation where the light emitting circuit near the drivingcurrent source module 21 is short-circuit grounded.

According to one embodiment, there is provided such a method to detectshort-circuit fault before startup of the light emitting device. Beforestarting the light emitting device and the driving apparatus, thedriving current source module 21 is disabled in step S611. As statedabove, those skilled in the art would appreciate that disable meansstopping use of a corresponding device or apparatus. For example, whenthe current source 21 is disabled, the current source may be made not tooperate by a control signal, or no power is supplied to the currentsource, or the current source is disconnected from other device,apparatus or circuit and the like so as to be electrically isolated fromother device, apparatus or circuit and the like.

Then, in step S613, a controller 41 controls a current injection circuit42 to inject current into a node N_(cs). The current, for example, maybe a small current for example, 100 μA. After continuous injection for awhile (e.g., 100 milliseconds), the controller 41 controls the currentinjection circuit 42 to stop injecting current.

In step S615, the controller 41 controls a voltage detection circuit 43to detect voltage on a node N_(cs). In another scenario, the controller41 may continuously receive voltage from a voltage detecting circuit 43.In a further example, the controller 41 may also be directly coupled tothe node N_(cs) to obtain the voltage on the node N_(cs), therebysparing the voltage detecting circuit 43. Besides, those skilled in theart may select the amount and duration of current injection based on thespecific conditions of the light emitting device and the drivingapparatus that are designed in actuality.

Next, in step S617, the controller 41 determines whether a voltageobtained on the node N_(cs) is smaller than a threshold V_(TH1) (e.g.,100 mV). Those skilled in the art may understand, the value of thethreshold V_(TH1) may be selected based on the specific conditions ofthe light emitting device and the driving apparatus that are actuallydesigned. When the voltage on the node N_(cs) is smaller than thethreshold V_(TH1), for example, the voltage on the node N_(cs) is 0V, itmeans the node N_(cs) is grounded. In other words, the LED near thedriving current source module 21 in the LED string is grounded.Therefore, in step S619, the driving apparatus is disabled. Disablingthe driving apparatus may comprise: disabling the current injectioncircuit 42, processor 41, voltage detecting circuit 43, boost module 3,and driving current source module 21, and the like, for example,disabling the driving module 4 or other circuits in the integratedcircuit chip.

However, when the voltage on the node N_(cs) is greater than or equal tothe threshold V_(TH1), it means the light emitting device does not havethe above short-circuit circumstance. The reason is that at this point,it is similar to the scenario of charging a capacitor; at the two endsof the light emitting device such as LED string, a certain voltageexists due to injection of current for a period of time. At this point,the device may start. Therefore, in step S6193, the controller providesa device-startup signal to start the driving apparatus and lightemitting device. Those skilled in the art may understand, at this point,open-circuit scenario may also possibly exist in a light emitting devicesuch as an LED panel. However, open circuit generally does not cause thedriving current source module 21 to operate uncontrollably and will notgenerally cause damage to the device; therefore, the driving apparatusand the light emitting device may still be started.

A flow of detecting whether faults exist in a light emitting devicebefore startup of the device has been described above. Hereinafter,examples of detecting whether a fault exists when the light emittingdevice is running will be described in detail with reference to FIG. 5and FIG. 6B.

With reference to FIG. 5 and FIG. 6B, when the driving apparatus and thelight emitting device are running, in step S601, the controller 41controls the voltage detection circuit 43 to continuously detect thevoltage on the node N_(cs). As shown in step S603, it is determinedwhether the voltage on the node N_(cs) is smaller than a thresholdvoltage V_(TH2). When the driving apparatus and the light emittingdevice operate normally, the voltage on the node N_(cs) is greater thanor equal to the threshold V_(TH2). At this point, the flow returns tostep S601 to perform detection. The threshold voltage V_(TH2) may beidentical to or different from the threshold voltage V_(TH1), which isdetermined dependent on the actual circuit design needs. When it isdetermined that the voltage on the node N_(cs) is smaller than thethreshold voltage V_(TH2), a fault occurs, and at this point, it needsto determine the fault type to perform corresponding processing.

Next, in step S621, similar to step S611, a driving current sourcemodule 21 is disabled. Next, step S623 of injecting current into thenode N_(cs), step S625 of detecting a voltage on the node N_(cs), stepS627 of determining whether the node N_(cs) is smaller than thethreshold are performed in sequence. Steps S621, S623, S625, and S627are similar to steps S621, S623, S625, and S627, respectively, whichwill not be detailed here.

After step S627, when it is determined that the node N_(cs) voltage issmaller than the threshold, for example, as shown in step S6291, thedriving apparatus is disabled, like in step S6191. When it is determinedthat the voltage on the node N_(cs) is greater than or equal to thethreshold, it indicates that the fault type is open circuit; therefore,it is allowed to only disable the driving current source module 21 andthe corresponding LED string, while other driving current source modules(such as the driving current source module 2 m) are not affected. Sincethe LED panel includes a LED array as shown in FIG. 2, the above stepsmay be performed in turns according to the LED string or performedsimultaneously. Besides, detection of the voltage on the runtime nodeN_(cs) may be performed continuously or as per a certain time interval.The advantage of continuous detection lies in real-time finding offaults, while the advantage of periodical detection lies in savingpower.

FIG. 7 shows another example of the protection circuit 4. In FIG. 7, theprotection circuit 4 comprises a current injection controller 401, atiming circuit 402, an injection current source 403, a first comparator404 and a second comparator 405. In this example, the voltage on thenode N_(cs) is detected continuously. The voltage is directly fed to aninverted end of the first comparator 404 so as to compare with thereference voltage V_(REF) received at the in-phase end of the firstcomparator 404. In this case, the abovementioned voltage detectingcircuit 43 is omitted. In other words, the voltage detecting circuit maycorrespond to the electrical connection between the node and the firstcomparator 404. The V_(REF) may be the above threshold voltage V_(TH2).When it is determined that the voltage on the N_(cs) is smaller thanV_(TH2), the first comparator 404 outputs a signal to disable thedriving current source module 21. Besides, the signal is also fed to thecurrent injection controller 401. The controller 401 starts theinjection current source 403 based on the signal so as to inject currentto the node N_(cs). Moreover, the current injection controller 401 alsostarts the timing circuit 402 such that the timing control circuit 402controls the time for current injection, e.g., 100 milliseconds.Although the current injection controller 401 and the timing controlcircuit 402 are shown as separate modules in FIG. 7, those skilled inthe art may understand that the current injection controller 401 may bedesigned to have a timing function, thereby sparing a separate timingcircuit 402. The timing circuit 402 outputs an enabling signal EN to thesecond comparator after a period of time, such that the secondcomparator 405 compares the node voltage as received at the inverted endwith the reference voltage V_(REF) as received in-phase. When the nodevoltage is smaller than the reference voltage V_(REF), the secondcomparator 405 outputs a signal of disabling the driving apparatus asmentioned above, while when the node voltage is greater than or equal tothe reference voltage V_(REF), the second comparator 405 outputs asignal of disabling the driving current source module 21 as mentionedabove. Another example of the protecting circuit 4 has been describedabove with respect to runtime detection and protection. Those skilled inthe art would understand that the protection circuit may also be used todetect circuit faults before starting the driving apparatus and thelight emitting device, e.g., the first comparator 404 is controlled tooutput a disable signal to the driving current source module 21 beforestartup of the driving apparatus and the light emitting device, whilethe output of the second comparator 405 may be used to indicate whethera short-circuit fault of the light emitting device exists. Besides,those skilled in the art may also understand that, other circuit designthan the protection circuits as shown in FIGS. 5 and 7 may also exist,as long as it can perform the method for detecting and protecting asshown in FIGS. 6A and 6B.

A driving apparatus for detecting a fault of a light emitting devicesuch as LED panel and a method for detection and protection has beendescribed above with respect to an embodiment. Complex detection circuitis not needed and interference brought by external circuits is thusavoided, since the protection circuit, the boost module, the drivingcurrent source and the like can be integrated into a same IC chip.Furthermore, the skilled in the art, upon review of the aboveembodiment, will appreciate that the driving apparatus for detecting afault of a light emitting device such as LED panel and a method fordetection and protection of the invention is simple in the sense ofcircuit design, and can provide real-time detection and protection tothe driving apparatus.

FIG. 8 shows configurations of a driving apparatus 5 and a lightemitting device according to another embodiment. In FIG. 8, for the sakeof illustration, only one LED string in the LED panel/array as shown inFIG. 2 is illustrated, which LED string is powered by the boost moduleand coupled to the driving apparatus 5 via the node N_(cs) so as to bedriven by the driving apparatus 5. Those skilled in the art mayunderstand that the node N_(cs) may be any point in the LED stringbetween the LED connected to the driving apparatus 5 and the currentsource, e.g., when the current source is integrated into the drivingapparatus while the LED string is integrated into the light emittingdevice, the node N_(cs) may be a pin of the driving apparatus connectedto the light emitting device. Those skilled in the art may understandthat the boost module and the driving current source module 5 in FIG. 8may be integrated into one integrated circuit chip, or located indifferent integrated circuit chips. Besides, the driving apparatus 8 mayinclude a protection module 4 as mentioned above. In order to make FIG.8 much clearer, the protection module 4 is not shown in FIG. 8. From theperspective of function, the driving apparatus 5 in FIG. 8 is similar tothe driving current source module 21 in the previous embodiment and canbe used in exchange in certain circumstances (e.g., in low-light state).When a light emitting device such as LED panel is in a low-light state(i.e., the LED panel is in a low-illumination brightness), the drivingapparatus always performs dimming in a pulse width modulation (PWM)mode. For example, with reference to FIG. 8, in a low-light state, adigital/analog converter (DAC) 51 supplies current I_(SET) to anoperational amplifier 52. Since only a very small part of currentI_(SET) flows into the in-phase end of the operational amplifier 52,which is so small that it can even be ignored, the current flowingthrough a resistor R2 is substantially equal to the current I_(SET). Theresistor R2 and one end of the resistor R1 are connected to the samepotential, e.g., grounded. Therefore, in this case, the voltage V+ atthe in-phase end of the operational amplifier 52 is substantially equalto R2*I_(SET). Since the voltages at two input ends of the operationalamplifier 52 are substantially equal, the voltage V− at the inverted endof the operational amplifier 52 is also substantially equal toR2*I_(SET). Likewise, the current flowing through the switch 53 issubstantially equal to the current flowing through the resistor R1.Therefore, the following equation (1) may be derived:

R2*I _(SET) =R1*I _(L)  (1)

wherein, I_(SET) is dimming current, while I_(L) is current flowingthrough the diode string. Therefore, dimming of the LED panel may berealized through changing I_(SET) (i.e., change the value of I_(L)). Theabove equation (1) may be revised as:

I _(L) =I _(SET) *R1/R2=I _(SET) *N  (2)

wherein N denotes the ratio between the resistance values of R1 and R2.

In a conventional PWM mode, N is a constant value. Therefore, dimmingthe LED panel is realized by changing the I_(SET) outputted by the DAC.As stated above, dimming in the conventional PWM mode has a delay, i.e.,the delay between the PWM signal and the I_(L) current, which causes thedriving current to be inaccurate, such that the illumination brightnessof the LED panel in the low-light state is also inaccurate, which,however, is not desirable.

Hereinafter, refer to FIG. 9 and FIG. 10, in which the operation anddelay of the LED in the PWM mode are presented. For the convenience ofillustration, FIG. 9 only illustrates the driving apparatus 5. Thecircuit configuration in FIG. 9 is substantially similar to referencenumeral 5 in FIG. 8, except that a first switch S1 and a second switchS2 controlled by the PWM signal are added in FIG. 9. The operation ofthe operational amplifier 52 and the on and off of the NMOS 53 arerealized through switching on and off of the first switch S1 and thesecond switch S2. Those skilled in the art may understand that theswitches S1 and S2 may be implemented by any device capable ofperforming a high-speed switch function such as MOS transistor, bi-polartransistor, and the like. The NMOS 53 is only an example, and it mayalso be implemented by a PMOS or a bi-polar transistor and the like.

Refer to FIG. 10. The PWM pulse signal is similar to a steppedsquare-wave signal, for controlling the on and off of the first andsecond switches S1 and S2. When PWM signal is 0, the first switch S1 isoff and the second switch S2 is on. At this point, the operationalamplifier 52 has no inverted end input, and the output of theoperational amplifier 52 is grounded, which causes off-state of the NMOS53; therefore, no current I_(L) flows through the resistor R1. When thePWM signal is stepped from 0 to 1, the S1 is on, and S2 is off; thecircuit at this point is similar to the circuit as shown in FIG. 8. Theoutput of the operational amplifier 52 is a high level, such that theNMOS 53 is conductive, and I_(L) flows through the resistor R1. Theaverage illumination brightness of the LED may be dimmed by adjustingthe duty ratio of the PWM. The studies show that in an actual operationprocess, there is always a delay between the stepped of the PWM signaland the current I_(L), e.g., referring to FIG. 10, a delay T_(d) existsbetween the PWM signal and the current IL, which causes the dimminginaccurate. Therefore, it is expected that delay can be reduced.

In the prior art, a more complex operational amplifier was designed toreduce the delay T_(d). This causes significant increase of themanufacturing costs of the driving apparatus and causes increase of thechip size. In the circuit configurations in FIG. 8 and FIG. 9, thestudies show that the delay T_(d) is mainly caused by the operationalamplifier 52. Further study shows that the response of the operationalamplifier 52 may be improved mainly through the following two manners:one being a burst mode, namely, promoting the current flowing through adifferential transconductance stage within the operational amplifier 52at the initial phase immediately following the PWM signal conversion(e.g., 200 nanoseconds) to improve the response of the operationalamplifier 52, thereby reducing the delay T_(d) between the PWM signalstep and the current I_(L); the other manner being boosting the voltageV_(set) at the input end of the operational amplifier when the currentflowing through the LED is too small, thereby improving the response ofthe operational amplifier 52.

Hereinafter, the two manners will be described, respectively.

FIG. 11 shows an exemplary operational amplifier according to oneembodiment. The operational amplifier is a typical folded cascodeoperational amplifier, comprising three parts: differentialtransconductance stage, current stage, and cascode current mirror load.In FIG. 11, the three parts are separated by dotted lines. As shown inFIG. 11, the differential transconductance stage is concatenated withthe current stage and immediately follows the cascode current mirrorload. The differential transconductance stage comprises a pair ofdifferential input transistors M1 and M2, with one ends being coupled tothe transistor M3 and the other end being coupled to the current stage.The current stage comprises transistors M4, M5, M6, and M7, coupledbetween a transconductance input stage and the cascode current mirror.The cascode current mirror comprises inter-coupled transistors M8, M9,M10, and M11. This kind of operational amplifier may further comprise acompensation circuit, such as Miller compensation. Those skilled in theart may understand that the folded operational amplifier in thisembodiment may be modified, and a part of devices or sub-circuits may beadded or deleted according to the actual requirement.

FIG. 12 shows an exemplary operational amplifier according to anotherembodiment. The operational amplifier is a folded operational amplifiercomprising a Miller compensation capacitor C0. The improvement of theembodiment in FIG. 12 over the embodiment in FIG. 11 lies in addingthree additional current source circuits. As shown in FIG. 12, in thedifferential transconductance stage, the ends of a pair of differentialinput transistors are coupled to a current source circuit which isfunctionally similar to the transistor M3 in FIG. 11, while the otherends are coupled to the current stage at the right side of thedifferential transconductance stage. In order to increase the responseof the operational amplifier, at one end of the differential inputtransistor, a first additional current source circuit coupled to thecurrent mirror circuit is added, which comprises one current source andone NMOS transistor, and at the other end of the differential inputtransistor, a second and a third additional current source circuits areadded, comprising one current source and one NMOS transistor,respectively. The three NMOS transistors are controlled by a burst_ensignal, respectively, so as to maintain a high level and last a certainperiod of time (e.g., 200 ns) upon PWM signal step (e.g., from 0 to 1,or from 1 to 0). When the NMOS transistor is conducted, the threeadditional current source circuits are conducted and provide additionalcurrent to co-operate, thereby increasing the charging speed for theMiller compensation capacitor C0. In this way, the output response ofthe folded operational amplifier in this embodiment is improved. Thoseskilled in the art should understand that other implementation mannersmay also be available, for example, increasing the current provided bythe current mirror circuit at one end of the differential inputtransistor in FIG. 2, and increasing the current of the current sourcein the current stage. Those skilled in the art may also understand thatthe above embodiments are only exemplary, not intended for restriction.For other type of operational amplifiers, their response may also bepromoted by increasing the internal current upon PWM signal step.

FIG. 13 shows a simulation diagram according to one embodiment. Theupper part of FIG. 13 depicts a PWM pulse signal, while the lower partof FIG. 13 depicts a current I_(L) curve with use of a burst mode and acurrent I_(L) curve without use of a burst mode (conventional mode) forcomparison therewith. FIG. 13 shows that, in the conventional mode, thedelay T_(d) is about 1.5 microseconds, while in the burst mode, thedelay T_(d) is about 75 nanoseconds, which is far less than the 1.5microseconds. It is seen that using the burst mode at the initial phaseof the PWM pulse signal conversion may greatly reduce the response delayof the operational amplifier, such that the LED panel dimming in alow-light state becomes more accurate (because the accuracy of thecurrent is higher). Besides, compared with the driving apparatus using amore complex operational amplifier, the design of the driving apparatusaccording to the embodiments is much simpler, which effectively reducesthe power consumption of the driving apparatus and also significantlyreduces the chip occupied area when it is completely integrated in thechip.

Another manner of improving the delay caused by the operationalamplifier involves boosting the voltage at the input end of theoperational amplifier when the current I_(L) flowing through the LED. Asstated above, and as shown in FIGS. 8 and 9, the studies show that theresponse of the operational amplifier may be improved by boosting thevoltage V_(set). Since the voltages at the in-phase end and the invertedend of the operational amplifier 52 when operations are substantiallyidentical, when the first switch S1 is on, the voltage at one end of R1connected to the first switch (suppose the other end of R1 is grounded)is equal to Vset; therefore, when boosting the Vset voltage, the voltageat one end of R1 connected to the first switch is also boostedaccordingly. For the circuit structures as shown in FIGS. 8 and 9, thecurrent I_(L) usually maintains unchanged, because it needs to guaranteethe accuracy of the LED panel illumination brightness. Therefore, theresistance value of the resistance R1 needs to be increasedcorrespondingly.

In one example, the resistor R1 comprises a plurality of seriallyconnected resistors, and switches connected in series with theresistors, respectively. For example, as shown in FIG. 14A, the resistorR1 comprises two resistors R11 and R12, and switches S_(R2) and S_(R1)connected in series therewith, respectively. When the I_(L) isrelatively low, the switch S_(R2) which was originally on is switchedoff, thereby increasing the resistance of the resistor R1. In anotherexample, the resistor R1 comprises a plurality of parallel connectedresistors and switches connected in series therewith, respectively. Forexample, as shown in FIG. 14B, the resistor R1 comprises two resistorsR12 and R11, and switches S_(R2) and S_(R1) connected in seriestherewith, respectively. When the I_(L) is relatively low, the switchS_(R2) which is originally on is switched off, thereby increasing theresistance of the resistor R1. Those skilled in the art may understandthat the above examples are only illustrative, not intended to limitthat the resistor R1 only includes R11 and R12. The resistor R1 maycomprise more serially connected resistors, and switches connected inparallel therewith, respectively. The resistor R1 may be implemented bypolycrystalline silicon, MOS transistor, and the like, while theswitches may be implemented by a high-speed switch such as MOStransistor or bi-pole transistor, and the like, so as to facilitateintegration.

On the other hand, for example, with reference to FIG. 9, because Vsetneeds to be increased, it can be implemented in two manners. One manneris correspondingly increasing Iset, and the other manner iscorrespondingly increasing the resistance of the resistor R2. A mannersimilar to increasing the resistance of resistor R1 as described withreference to FIGS. 14A and 14B may be used to increase the resistance ofthe resistor R2. In one example, when I_(L) is relatively low, theresistance of the resistor R1 and that of the resistor R2 are increasedwith the same proportion, thereby maintaining a proportion of 1:N. Inanother example, when the I_(L) is relatively low, the resistance of theresistor R2 is increased, and Iset is increased, but the product ofmultiplication of the increase proportion of the resistor R2 and that ofthe current Iset is equal to the increase proportion of the resistor R1.In a further example, when I_(L) is relatively low, the current value ofIset is decreased, while the resistance of the resistor R2 is increased,but the product of multiplication of the increase proportion of theresistor R2 and the decrease proportion of the current Iset is equal tothe increase proportion of the resistor R1. In a still further example,when I_(L) is relatively low, the current value of Iset is increased,while the resistance of the resistor R2 is decreased, but the product ofmultiplication of the decrease proportion of the resistor R2 and theincrease proportion of the current Iset is equal to the increaseproportion of the resistor R1.

Generally speaking, the method according to one embodiment may improvethe response of the operational amplifier by shortly (e.g., 200 ns)increasing the current flowing through the differential transconductancestage within the operational amplifier upon PWM signal conversion (e.g.,from low to high or from high to low) when performing dimming to a lightemitting device such as LED panel in a PWM dimming mode. The methodaccording to another embodiment may improve the response of theoperational amplifier by boosting the voltage at the input of theoperational amplifier in the driving apparatus and increasing theresistance of the resistor through which the driving current flows whenI_(L) is relatively low.

Increasing the resistance of the resistor unit through which the drivingcurrent flows may be realized by using the resistors as described abovewith reference to FIGS. 14A and 14B. Boosting the voltage at the inputmay be realized by changing the resistance value of another resistorunit coupled to the input of the operational amplifier or changing thecurrent flowing through said another resistor unit.

Those skilled in the art may understand that the above two methods ofimproving the response of operational amplifier may be usedindividually, or in combination to achieve a better effect. Thoseskilled in the art may understand, the LED panel/array used in the aboveembodiments are only for illustration, rather than for limitation. Othertypes of light emitting devices may also apply here. Besides, thedriving apparatus and light emitting device involved here and thecorresponding methods may also be applied to various electronic devicesemploying such as LED panel/array, for example, a liquid crystaldisplay, a LCD TV, a computer, a mobile phone, a player, or a personaldigital assistant. For example, in a mobile cellular phone, theprocessor may control a driving apparatus according to the embodimentswhich may be included in a mobile cellular phone according toinstructions stored in the memory, to drive a display screen of themobile cellular phone. Besides, those skilled in the art may understand,various apparatuses, devices and modules and the like as stated hereinmay be implemented by a circuit, an integrated circuit, or a chip.Therefore, these terms may be used in exchange and have similar oridentical meanings.

Some embodiments have been described above. In general, according to oneembodiment, there is provided a driving apparatus for driving a lightemitting device, comprising: a driving current source module configuredto supply current to the light emitting device via a node duringoperation; a protection module coupled to the node and the drivingcurrent source module and configured to selectively inject current tothe node during operation and control the driving current source modulebased on a detection result of a voltage on the node.

In the driving apparatus, the protection module is further configured tocontrol a boost circuit coupled to the light emitting device, whereinthe boost circuit is for supplying voltage to the light emitting device.

In the driving apparatus, the protection module is further configured todisabling the driving current source module before startup of thedriving apparatus; inject current into the node; detect a voltage on thenode; and enable or disable the driving apparatus based on the detectedvoltage.

In the driving apparatus, the protection module is further configured todisable the driving apparatus when the detected voltage on the node issmaller than a threshold voltage; and enable the driving apparatus whenthe detected voltage on the node is greater than or equal to thethreshold voltage.

In the driving apparatus, the production module is further configured todetect a voltage on the node when the driving apparatus is running, anddisable the driving current source module when it is detected that thevoltage on the node is smaller than a threshold voltage; inject currentinto the node; detect a voltage on the node; and select disabling thewhole driving apparatus or only disabling the driving current sourcemodule based on the detected voltage.

In the driving apparatus, the protection module comprises: a currentinjection circuit configured to selectively inject current into thenode; a voltage detection circuit configured to selectively detect avoltage on the node; and a controller configured to selectively controlthe driving apparatus, the current injection circuit, and the drivingcurrent source module.

In the driving apparatus, the protection module further comprises atiming circuit configured to control time when the current injectioncircuit injects current into the node.

In the driving apparatus, the controller comprises a first comparatorand a second comparator, wherein the first comparator selectivelytransmits a first comparator output to the current injection circuit andthe timing circuit based on output of the voltage detection circuit anda reference voltage; the second comparator selectively outputs a secondcomparator output for disabling the driving apparatus or the drivingcurrent source module based on output of the voltage detection circuit,the reference voltage and output of the timing module.

According to one embodiment, there is provided a display system,comprising a light emitting device and the driving apparatus for drivingthe light emitting device.

According to one embodiment, there is provided an electronic devicecomprising a display system.

The electronic device is a display, a LCD TV, a computer, a mobilephone, a player, or a personal digital assistant.

According to one embodiment, there is provided a method for protecting adisplay system, which display system comprises a light emitting deviceand a driving apparatus for driving the light emitting device, whichlight emitting device is coupled to a current source of the drivingapparatus via a node, which method comprises: disabling the drivingcurrent source module before startup of the driving apparatus; injectingcurrent into the node; detecting a voltage on the node; and determiningwhether to enable or disable the driving apparatus based on the detectedvoltage.

In the above method, if the detected voltage is smaller than a thresholdvoltage, the driving apparatus is disabled; if the detected voltage isgreater than or equal to the threshold voltage, the driving apparatus isstarted.

According to one embodiment, there is provided a method for protecting adisplay system, which display system comprises a light emitting deviceand a driving apparatus for driving the light emitting device, whichlight emitting device is coupled to a current source of the drivingapparatus via a node, which method comprises: detecting voltage on thenode when the driving apparatus is running; disabling the drivingcurrent source module when it is detected that voltage of the node isless than a threshold voltage; injecting current into the node;detecting a voltage on the node; and selecting only disabling thedriving current source module or disabling the entire driving apparatusbased on the detected voltage.

In the above method, if the detected voltage is smaller than a thresholdvoltage, only the driving current source module is disabled; if thedetected voltage is greater than or equal to the threshold voltage, theentire driving apparatus is disabled.

In the above method, when the driving apparatus is running, voltage onthe node is detected continuously or periodically.

According to one embodiment, there is provided a driving apparatus fordriving a light emitting device, comprising: a driving current sourcemodule configured to supply current to the light emitting device via anode during operation, wherein the driving current source module isconfigured to improve dimming accuracy by reducing delay caused by anoperational amplifier included in the driving current source module whendimming in a pulse width modulation (PWM) mode.

In the driving apparatus, the operational amplifier comprises a firstinput, a second input, and an output. Besides, the driving currentsource module further comprises: a second resistor unit coupled betweenthe first input and a ground; a first switch coupled to the secondinput; a first resistor unit coupled between the first switch and theground; a second switch coupled between the output and a ground; aswitch unit coupled between the light emitting device and the firstresistor unit, which switch unit is configured to be controlled by theoutput to selectively conduct; and a controller, wherein the controlleris configured to boost voltage at the first input and increase aresistance value of the first resistor unit.

In the driving apparatus, the controller is further configured tocontrol to boost voltage at a first input of the optional amplifierthrough increasing a resistance value of the second resistor unit.

In the driving apparatus, the second resistor unit comprises a pluralityof serially connected resistors, and a plurality of switches in parallelconnection with respective resistors, wherein the controller isconfigured to increase a resistance value of the second resistor unitthrough selectively disconnecting a switch in the plurality of switches.

In the driving apparatus, the second resistor unit comprises a pluralityof parallel connected resistors, and a plurality of switches in seriesconnection with respective resistors, wherein the controller isconfigured to increase the resistance value of the second resistor unitby selectively disconnecting a switch in the plurality of switches.

In the driving apparatus, the controller is configured to control toboost voltage at a first input of the operational amplifier throughincreasing current flowing through the second resistor unit.

In the driving apparatus, the first resistor unit comprises a pluralityof serially connected resistors, and a plurality of switches parallelconnected to respective resistors, wherein the controller is configuredto increase the resistance value of the first resistor unit byselectively disconnecting a switch in the plurality of switches.

In the driving apparatus, the first resistor unit comprises a pluralityof parallel connected resistors, and a plurality of switches seriallyconnected to respective resistors, wherein the controller is configuredto increase the resistance value of the first resistor unit throughselectively disconnecting a switch in the plurality of switches.

In the driving apparatus, the controller is configured such thatproportion of voltage boost at the first input is identical to theproportion of increase of the resistance value of the first resistorunit, such that the current flowing through the first resistor unitkeeps unchanged.

In the driving apparatus, the operational amplifier comprises adifferential transconductance stage, and wherein the delay is reduced byincreasing the current flowing through the differential transconductancestage upon PWM signal step.

In the driving apparatus, the operational amplifier further comprises aMiller compensation capacitor, wherein the delay is reduced by supplyingpower to the Miller compensation capacitor from increased currentflowing through the differential transconductance stage upon PWM signalstep.

In the driving apparatus, the differential transconductance stagecomprises a pair of differential input transistors, one ends of whichdifferential input transistors are coupled to a current mirror circuit,while the other end thereof is coupled to a current stage, wherein thedifferential transconductance circuit further comprises a firstadditional current source circuit connected in parallel to the currentmirror circuit and a second additional current source circuit and athird additional current source circuit, which are coupled to the otherends of the pair of the differential input transistors, the firstadditional current source circuit, the second additional current sourcecircuit, and the third additional current source circuit beingconfigured to supply additional current upon PWM signal step.

In the driving apparatus, the increased current flowing through thedifferential transconductance stage lasts a predetermined period of timeupon PWM signal step.

According to one embodiment, there is provided a display systemcomprising a light emitting device and the driving apparatus for drivingthe light emitting device.

According to one embodiment, there is provided an electronic devicecomprising a display system.

The electronic device is a display, a LCD TV, a computer, a mobilephone, a player, or a personal digital assistant.

According to one embodiment, there is provided a method for driving alight emitting device, comprising: improving dimming accuracy byreducing delay caused by an operational amplifier in the drivingapparatus when dimming in a pulse width modulation (PWM) mode.

The method comprises reducing the delay through increasing an inputvoltage of the operational amplifier.

The method comprises reducing the delay by increasing current flowingthrough a differential transconductance stage in the operationalamplifier upon PWM signal step.

Although the present invention has been disclosed with reference to aplurality of preferred embodiment, it should be understood that thepresent invention is not limited the disclosed preferred embodiments.The present invention intends to cover various modifications andequivalent arrangements within the spirit and scope of the appendedclaims. The scope of the appended claims conforms to the broadestexplanations, thereby including all such modifications and equivalentstructures and functions.

What is claimed is:
 1. A method for driving a light emitting device,comprising: supplying a first current over a first current path directlyto a node of said light emitting device to cause light production; andselectively injecting a second current over a second current pathdirectly to said node of said light emitting device during a testingoperation; detecting a voltage at said node; and controlling the supplyof the first current in response to the detected voltage at said node.2. The method according to claim 1, further comprising: disabling supplyof the first current before entering a startup mode of operation; injectthe second current to the node; detecting the voltage at the node; andselectively supplying of the first current in response to the detectedvoltage.
 3. The method according to claim 3, further comprising:disabling operation of the light emitting device when the detectedvoltage is smaller than a threshold voltage; and enabling operation ofthe light emitting device when the detected voltage on the node isgreater than or equal to the threshold voltage.
 4. The method accordingto claim 1, further comprising: detect the voltage at the node while thefirst current is being supplied; disabling supply of the first currentwhen the detected voltage is smaller than a threshold voltage; injectingthe second current into the node; detecting an additional voltage at thenode; and selectively disabling either operation of the light emittingdevice or supply of the first current based on the detected additionalvoltage.
 5. The method according to claim 4, wherein selectivelydisabling comprises: disabling operation of the light emitting devicewhen the detected additional voltage is smaller than a thresholdvoltage; and disabling supply of the first current the driving currentsource module when the detected voltage on the node is greater than orequal to the threshold voltage.
 6. A method for protecting a displaysystem which includes a light emitting device and a driving apparatusfor driving the light emitting device, wherein the driving apparatusincludes a current source coupled to supply a drive current to the lightemitting device, the method comprising: disabling the current sourcebefore a startup of the driving apparatus; injecting a test current intoa node of the light emitting device; detecting a voltage at the node;and determining whether to enable or disable the driving apparatus basedon the detected voltage.
 7. The method according to claim 6, whereindetermining comprises: disabling the driving apparatus if the detectedvoltage is smaller than a threshold voltage; and enabling the drivingapparatus if the detected voltage is greater than or equal to thethreshold voltage.
 8. The method according to claim 6, wherein the drivecurrent is supplied to the node over a first current path and the testcurrent is supplied to the node over a second current path differentfrom the first current path.
 9. A method for protecting a display systemwhich includes a light emitting device and a driving apparatus fordriving the light emitting device, wherein the driving apparatusincludes a current source coupled to supply a drive current to a node ofthe light emitting device, the method comprising: detecting a voltage atthe node when the driving apparatus is operating; disabling the currentsource when the detected voltage at the node is smaller than a thresholdvoltage; injecting a current into the node; detecting a further voltageat the node; and selectively disabling only the current source ordisabling the driving apparatus based on the detected further voltage.10. The method according to claim 9, wherein selectively disablingcomprises: disabling only the current source if the detected furthervoltage is greater than or equal to the threshold voltage; and disablingthe driving apparatus if the detected voltage is smaller than thethreshold voltage.
 11. The method according to claim 9, whereindetecting the voltage at the node when the driving apparatus isoperating comprises making the detection on a continuous basis.
 12. Themethod according to claim 9, wherein detecting the voltage at the nodewhen the driving apparatus is operating comprises making the detectionon a periodic basis.
 13. A method for driving a light emitting device,comprising: improving dimming accuracy by reducing a delay caused by anoperational amplifier in the driving apparatus upon dimming in a pulsewidth modulation (PWM) mode.
 14. The method according to claim 13,comprising reducing the delay by increasing an input voltage of theoperational amplifier.
 15. The method according to claim 13, comprisingreducing the delay by increasing current flowing through a differentialtransconductance stage in the operational amplifier upon PWM signalstep.